Two-part endosseous dental implant systems for insertion in a wholly or partially edentulous region of the jawbone of a patient have been in use for a few years. These implant systems may be completely embedded in the jawbone, covered with mucosal tissue, and permitted to remain in place while new bone grows around the implant, and into and/or through one or more vent holes in the implant itself. Once the implant has become firmly anchored in bone, the mucosal tissue is reopened, and an abutment or post is connected to the implant using a screw. A prosthesis can then be connected to the abutment or post.
Many of these two-part implant systems have, at the top end of the implant, an external, hexagonal projection, sometimes called a male hex, which projects upwardly from the top end of the implant, leaving a shoulder surrounding the base of the male hex. An abutment or post having an outer diameter that substantially matches the outer diameter of the implant is seated on the male hex to form a substantially sealed connection. Some of these implants have an externally-threaded, sidewall portion which can be screwed into an opening formed in bone after bone tissue has been removed from the jawbone, as depicted, for example, in U. K. Patent No. 1,291,470 or in U.S. Pat. No. 4,713,004. With implant systems of this kind, the male hex projection at the top of the implant is used to insert the implant in the jawbone, using an inserting device, e.g. a wrench, which is attached to the male hex.
Another kind of two-part, endosseous dental implant system with an external male hex is a cylindrical implant with a non-threaded, external body portion. These implants are pushed into an opening formed in bone tissue. An example of this type of implant is Core-Vent Corporation's BIO-VENT implant.
In implants of these kinds with external male heads, the male head is used to attach the implant to an abutment or post having a matching female, hex-shaped cavity that receives and engages the male hex projection. Such male hex heads and female hex cavities are sometimes referred to as coupling surfaces. The implant systems that now have such external hexes for coupling with an internal hex cavity on an abutment have the walls of the hexagonal head of the implant and the hex-shaped cavity of the abutment perpendicular and parallel to one another.
With such implant systems, the male hex of the implant is smaller in diameter than the diameter of the hex-shaped cavity of the abutment to permit the male hex to fit inside the female cavity. This difference in diameter is sufficiently large to allow for manufacturing variations while still allowing the coupling surfaces of the abutment to seat fully on the shoulder of the implant to create a sealed outer margin between abutment and implant. However, this leaves space between the coupling surfaces of the male and female hexes.
Within the hex head region, and extending into the implant itself, there is in such implants a threaded hole for receiving an attachment screw of a mating abutment. The abutment typically has an interior passage centered on its hex cavity. When attaching the abutment to the implant, the screw is inserted through the abutment passage and is screwed into the threaded implant hole. Tightening the screw also tightens the abutment against the implant. When the screw is tightened until the external hex of the implant mates with the matching female hex cavity in the abutment, the system is secured against axial displacement of the abutment from the implant.
According to reported studies, the seating of the external hex of the implant within the female hex cavity of the abutment, where both the external hex and the internal hex cavity have parallel walls, results in the full seating of the abutment onto the shoulder surrounding the external male hex of the implant, but fails to prevent rotational displacement of the implant with respect to the abutment.
One scientific study presented by Dr. Paul Binon at the Academy of Osseointegration meeting in San Diego, Calif. in March, 1993, documented that the coupling surfaces of commercially-available implants of these kinds have four to five degrees of rotation between them. Dr. Binon later reported that the Branemark implant/abutment assembly of this kind exhibits up to nine degrees of rotation between the implant and the abutment. The attachment between abutment and implant formed in this way is unstable. Lateral forces from biting are transmitted to the screw joining the abutment to the implant rather than the coupling surfaces of the external hex projection on the implant and the internal hex cavity in the abutment. As a result, the screw that joins the implant to the abutment may break or loosen. Rotational instability may also adversely affect the accuracy of transfer procedures needed for the indirect fabrication of a final prosthetic restoration on such implant/abutment assemblies.
U.S. Pat. No. 4,547,157 discloses an implant having a conical projection for mating with an abutment having a matching cavity. A small degree of taper of the two surfaces results in a friction fit between the parts that tends to maintain the connection. These systems do not use a screw that passes through the abutment to lock the abutment to the implant. In this implant/abutment connection system, no shoulder exists on the head of the implant for the outer edge of the walls of the internal cavity to rest upon and seal. With this tapered, cylindrical coupling surface making direct contact on full seating of the abutment in the implant, a good connection results. However, this type of connection results in a ledge being formed as the outer walls of the internal cavity fit over the conical projection of the implant. This ledge can trap food particles and irritate the gum tissue. Moreover, because the projection and mating cavity are conical, they provide little resistance to rotational forces that tend to loosen the connection.